1. Field of the Invention
This invention relates to underdrains in gravity filters and, more particularly, to caps for underdrain blocks.
Gravity filters frequently employ underdrain systems or filter bottoms for collecting and channelling effluent away from the filter bed and also for distributing backwash fluids into the filter bed. Such a filter bottom is described in U.S. Pat. No. 4,065,391 to Farabaugh. The filter bottom usually includes several filter underdrain blocks arranged in several parallel rows across the filter bottom. The underdrain blocks provide the appropriate conduits and apertures for receiving effluent and discharging backwash fluids. Typically, the underdrain blocks have several apertures in the top wall of the blocks for this purpose.
It has been found desirable to employ larger apertures in the top wall of the blocks so that the apertures will not clog and inhibit flow to and from the filter bed. A problem with the larger apertures is penetration of fine grain filter media, such as sand, anthracite or granular activated carbon ("GAC"), through the apertures and into the underdrain blocks. This eventually clogs the underdrains, and the loss of filter media hinders the filtering capabilities of the bed. The present remedy for this problem is employment of a support gravel barrier between the fine grain filter media and the underdrain blocks. However, this solution has several problems.
First, the support gravel is very expensive and time consuming to install as it requires several individual layers of gravel. The gravel is laid down in an hour-glass configuration so that a marked gradation from larger gravel to smaller gravel and back to larger gravel is maintained. This support barrier generally consumes 12 inches of filter space, which is particularly a problem when retrofitting low-clearance filters with new, non-clogging underdrain blocks. Second, the gradation in the support barrier is subject to upset when "uncontrolled air" enters the underdrain and comes out the apertures in the top walls of the underdrain blocks. The uncontrolled air may result from defects in the installation of the air system, or from operator error during normal operation. Finally, in GAC installations, the GAC must be occasionally removed from the filter and placed in a reactivation furnace. The removal is typically accomplished by flooding, and in the flooding process, support gravel frequently mixes with the GAC and accompanies it into the reactivation furnace. At the high temperatures necessary to reactivate the GAC, over 1400.degree. F., the gravel explodes and causes damage to the interior of the furnace.
FIG. 1 is a cross section of a prior art gravity filter A having a bottom wall B and two side walls C. Bottom wall B has a plurality of underdrain blocks D aligned end to end in several parallel rows across the bottom wall B. FIG. 1 shows one of these rows having four underdrain blocks D.
A filter bed E is placed on top of the underdrain blocks D within side walls C. Filter bed E includes a fine grain layer F and a support gravel layer G. The fine grain layer F may comprise sand, anthracite or GAC. The average diameter of the sand particles ranges from 0.25 to 0.50 mm., and that of the GAC ranges from 0.9 to 1.4 mm.
As noted above, the support gravel layer G is installed to keep particles from the fine grain layer F from draining into the underdrain blocks D. To accomplish this purpose, the support gravel layer G is installed in an hour-glass configuration, typically as follows:
2 inch depth of 3/4 inch by 1/2 inch gravel PA0 2 inch depth of 1/2 inch by 1/4 inch gravel PA0 2 inch depth of 1/4 inch by 1/8 inch gravel PA0 2 inch depth of 1/8 inch by #10 gravel PA0 2 inch depth of 1/2 inch by 11/4 inch gravel PA0 2 inch depth of 3/4 inch by 1/2 inch gravel
As stated, this layer is time consuming and expensive to install. Additionally, it consumes 12 inches of filter space and poses a significant head loss, which must be overcome to properly backwash the fine grain layer F.
2. Description of the Prior Art
U.S. Pat. No. 2,154,167 to Jenks entitled "Water Treatment" ("Jenks") discloses improvements in water treatment related to a sand filter bed under which a combined underdrain and distribution structure is located. The underdrain includes parallel channels separated from the bed by ceramic units having parallel ribs. The ceramic units are made from sintered, igneous materials, and they are said to have a permeability such that the head loss therethrough is small compared to the head loss through the conventional rapid sand filter system. A problem with the ceramic units is that their pores are easily calcified because the microscopic grains in these units have jagged, sharp edges. The sharp edges also tend to collect particulates both from the fluid being filtered and from the backwash fluid, further clogging the underdrain. Additionally, the rigidity of the ceramic units gives them a tendency to fail when clogged or subjected to uncontrolled high pressure "kicks" The ceramic units are also quite heavy and are thus difficult to handle.
A further problem with the ceramic unit underdrain disclosed in Jenks is its inherent inability to provide proper distribution of backwash fluids, particularly backwash gas. The nature of the manufacturing process makes it impractical to produce a ceramic unit of absolute uniform porosity so that head loss variations will be present across the continuum of the Jenks underdrain. Gas is very sensitive to such head loss variations, and when pumped into the underdrain under pressure, the gas will concentrate at locations of higher porosity (lower head loss) resulting in maldistribution of the gas into the filter bed.
The Jenks underdrain is further inhibited from properly distributing backwash fluids because it cannot practically be installed at true level. Typical underdrains vary as much as 1/8 inch from level across the length of the underdrain. As a result, incoming backwash gas collects at the high end of the underdrain, and distribution into the filter bed is therefore uneven. This problem is enhanced by the fact that the channels in Jenks are in fluid communication across the full length of the underdrain. The only metering of gas which occurs is between the channels and the filter bed itself, through the pores of the ceramic units.
U.S. Pat. No. 3,840,117 to Ross entitled "Underdrain for Waste Effluent Filter and Method of Using Same" ("Ross") discloses an underdrain structure for a waste effluent filter having an air and liquid distributing device in the underdrain cavity. The underdrain supports a sand bed, and the underdrain includes a plate having a plurality of apertures therein. Above the plate are upper pockets, defined by rims in the form of a matrix of plastic bars. A mesh screen is placed between the filter media and the plate, and the screen rests on the rims to further define the upper pockets. A central support sleeve is located within each upper pocket to support the screen, and a grate rests on top of the screen. The filter bed does not include gravel support layers. The Ross underdrain is relatively complex and time consuming to install. The wire mesh screen is subject to calcification. The matrix of plastic bars, the support sleeves and the grate effectively shield backwash fluids from contacting a large portion of the filter bed. This creation of "dead spots" detracts from the useful life of the filter bed and promotes undesirable build-up of filtrate.
U.S. Pat. No. 4,208,288 to Stannard, et al. entitled "Filtering Apparatus" discloses a filtering apparatus having rigid filter plates encasing a secondary filter zone. The filter plates are made from organic resin matrix with anthracite aggregate.
Thus, it is an object of the present invention to provide a cap for underdrain blocks which resists calcification and which is easy to install, both on existing underdrain blocks or in combination with newly manufactured blocks. It is a further object of the invention to enhance distribution of backwash fluids evenly across the entire filter bed to avoid the development of dead spots within the filter bed wherein the bed is not adequately cleaned. It is a still further object of the present invention to eliminate the need for a separate layer of support gravel, thus providing added clearance above the filter bed, reducing installation costs and eliminating filter media contamination by support gravel. Finally, it is an object of the present invention to provide a porous underdrain barrier which has a head loss smaller than the support gravel layer which it replaces, thus reducing the energy required in backwash operations.